Flyby anomaly indicates the existence of an unknown perturbation (i.e., anomalous acceleration) that affects hyperbolic trajectories. Based on the analytical position and velocity variations, this paper investigates the general kinematics of perturbed hyperbolic orbits. As a result, post-interaction approximation formulas are derived. Based on these results, the observation data of the Galileo and NEAR Earth flybys are analyzed. The analysis results derive new constraints for flyby kinematics. The authors of this paper selected a few of the hypothetical acceleration models and analyzed their kinematical properties as representative examples. The simulation results show that the acceleration models fail to reproduce the characteristics of the range and Doppler observation data. This means that, in modeling the flyby anomaly, not only energy variation, but also kinematical constraints must be considered to reproduce the observation data.
A GNSS fault detection method using an adaptive fading Kalman filter is proposed for detecting GNSS faults such as step- and ramp-type bias error in pseudorange measurements. The fading factor of the filter is used as a detection parameter. In order to detect the GNSS fault signals regardless of the GNSS fault type, the proposed method is based on a single base station that has a fixed location and is already known. In the simulations, the different fault signal types are represented by the ramp bias error and the step bias error of the pseudorange. The change in the fading factor according to the bias error is quantitatively analyzed, and a detection threshold is established to detect the GNSS fault signal by analyzing the change in the error covariance. In addition, the value of the fading factor is applied to adjust the Kalman gain and the effect of the fault signal is mitigated by controlling the Kalman gain of the filter. The performance of the proposed fault detection method is evaluated by simulations, and the results thereof confirm that this method can detect each channel affected by fault signals when GNSS faults occur.
The paper considers the ant colony optimization (ACO) methodology for designing an active debris removal mission. The goal is to optimize a sequence of transfers using an orbital transfer vehicle to rendezvous with multiple pieces of debris for the purpose of removal. The methodology consists of two phases: the first phase obtains an optimal removal sequence, and the second phase is related to transfer trajectory optimization. During the sequence planning process, a refined approximation is proposed to estimate the transfer times and necessary costs of individual transfers. The problem can then be mapped into a variant of the traveling salesman problem (TSP). To solve it, an enhanced ACO and the inver-over algorithm are proposed. The effectiveness of the ACO heuristic is proved over a set of instances with different sizes ranging from 100 to 2000. In the second phase, each transfer leg in the optimal sequence is verified using the continuous ACO proposed. The computational results show that the methodology proposed can optimally select targets from a debris archive of considerable size (i.e., up to 2000 debris pieces). Additionally, the mitigation of 13–20 objects, with total ΔV below 1 km/s, is feasible in less than a year.
A computational method capable of calculating radiative heat transfer in a porous carbon-based material is developed in this study. In the method, a radiative conductivity value is statistically evaluated using a three-dimensional model of the material constructed using X-ray computed tomography images. The method is applied for thermal response analysis of a non-ablative heat shield called the Non-Ablative Lightweight Thermal protection system (NALT), which is heated in an arcjet flow. The arcjet heating environment is calculated in a coupled manner between nonequilibrium arcjet flow computation and NALT thermal response analysis. The results shows that the computational method developed satisfactorily reproduces the surface and in-depth temperature data measured.
An experimental system for lodging a metal anchor into free-falling targets has been developed and the lodging behavior has been investigated. Lodging a metal anchor into space debris to capture it is a good candidate for gathering the debris in space debris mitigation systems. Being one of the simplest methods, it can be applied to most any system, such as the electrodynamic tether system or a propulsion system. Basically, a tethered anchor is fired into space debris to capture it. In our experimental setup, a test plate representing the target space debris is initially attached to a rigid frame using two electromagnetic mounts. A specially designed projectile anchor is fired into the target, in precise coordination with the release of the target test plate into a free-fall condition (i.e., controlled by optical sensors mounted on the acceleration tube and electromagnetic mount). As a result, the metal anchor impacts the target while free-falling through space. The experimental results show that an adequate anchor accuracy can be achieved with the appropriate projectile velocity for a non-fixed (free-falling) target. Moreover, we show that the penetration velocity differs predictably between fixed and non-fixed targets, as well as that predictability changes based on the distance of the projectile's impact point from the target's center-of-mass (CM). The equations derived for predicting penetration velocities for free-falling targets were validated through physical testing and the required penetration velocities were accurately predicted.